1. Technical Field
The present invention relates to a Hybrid Fiber-Coax [HFC] telecommunication system including a plurality of modulation means and a combiner means coupled thereto for receiving therefrom modulated signals, said combiner means being connected to a coaxial cable for transmitting thereon said modulated signals in predetermined frequency channels, different for each of said modulation means, each of said modulation means including signal mixer means having a first input connected to a quadrature signal generator adapted to supply a carrier signal thereto, a second input at which an analog telecommunication signal is applied, and an output coupled to said combiner means and at which the modulated signal is provided as a result of a modulation, in said signal mixer means, of said carrier signal by said analog telecommunication signal.
2. Discussion of Related Art
Such a Hybrid Fiber-Coax telecommunication system is generally known in the art. The idea of HFC is to put different telecommunication signals, received via optical fibers, onto distinct video channels and to apply these channels to the coaxial cable, preferably according to the Frequency Division Multiple Access [FDMA] structure. Each channel is modulated on a different carrier signal of which the frequency is in the video band. The outputs of the signal mixer means are generally connected to distinct inputs of the combiner means via drivers. The combiner means sums up the signals at the outputs of the drivers, i.e. sums up the voltages of the modulated signals.
A combiner means comprises at least one combiner as for instance known from Section 2: xe2x80x9cPower Splitter/Combinersxe2x80x9d of the xe2x80x9cRF/IF Designer""s Handbookxe2x80x94Mini-Circuitsxe2x80x9d edition 92/93 by Scientific Components. In general, a resistive or inductive combiner with an auto-transformer is used per two input signals, i.e. for two modulation means. Several of such combiners are then build in cascade so as to constitute a combiner means adapted to accept up to 12 voltage-mode input signals on a 50 or 75 Ohm interface. For each of these combiners, an auto-transformer introduces a 3 dB attenuation per two signals summed up, while a resistive combiner introduces a 6 dB attenuation.
This loss is a problem for the power of the above drivers. In fact, the driver is a power amplifier of which all the power is dissipated in the combiner connected thereto.
Another problem of the known HFC telecommunication system is the linearity requirement that is generally critical at the high frequencies wherein the system is used.
An object of the present invention is to provide a Hybrid Fiber-Coax telecommunication system of the known type but wherein the power consumption is reduced and the linearity is optimized.
According to the invention, this object is achieved due to the fact that said signal mixer means of each modulation means are adapted to provide, as modulated signal, an output current, and that said combiner means is a current summing node for the output currents of the signal mixer means of said plurality of modulation means.
The invention thus comprises in operating the combiner means as a current-mode summing node instead of as a voltage-mode summing node. The implementation of a current summing node is relatively easy and its linearity is higher than that of a voltage summing node. Furthermore, drivers or power amplifiers are no longer required because there is no power dissipated in such a combiner means, thus no power loss to be compensated. Additionally, the integration volume of the HFC telecommunication system is reduced, as well as the cost for an external combiner.
Another characteristic feature of the present invention is that said current summing node is a junction point coupled to the outputs of said signal mixer means and to said coaxial cable.
Summing of currents just needs to short the outputs of the current sources, i.e., in the present application, the signal mixer means. By reducing the combiner means to a single junction point, the linearity of the system is obviously optimized.
Also another characteristic feature of the present invention is that each of said signal mixer means includes a pair of mixer circuits to which quadrature carrier signals and quadrature analog telecommunication signals are applied via said first and second input respectively.
The present invention is thereby applicable to telecommunication systems of the quadrature-phase modulation type.
In a variant of the present invention, the outputs of the mixer circuits of each of said modulation means are coupled to the first mentioned current summing node via the cascade connection of a second current summing node and a current amplifier.
In some applications, current amplifiers may be necessary, for instance if the output currents of the mixer circuits are insufficient to be directly applied to the coaxial cable.
As the telecommunication signal provided to the modulation means is generally a digital signal, the present invention is also characterized in that said modulation means further includes digital-to-analog converter means connected to said second input of said signal mixer means and adapted to receive the telecommunication signal in a digital form and to convert it into an analog form prior to apply it to said signal mixer means.